Known optical transmission systems can be broadly categorized as direct detection, or coherent detection systems. In direct detection systems, at the receiver, the signal power is measured and therefore any phase and polarization information in the optical signal is ignored and lost. In coherent detection systems, inphase and quadrature components of the optical field are detected, which enables the use of phase modulation as well as amplitude modulation, and so two channels can be transmitted having orthogonal phases. It is also known that two further channels can be transmitted using orthogonal polarizations, if in addition the receiver is polarization diverse. Direct detection systems have nevertheless dominated the market for long haul transmission systems due to their simplicity. In contrast coherent receivers require careful polarization alignment and phase tracking, which is difficult and can limit the cost/performance trade off. In typical systems, the polarization may change at rates up to kHz levels, while phase variations can be typically up to MHz levels.
Both coherent and direct detection systems are also limited in high capacity systems by distortions introduced by the optical path, mostly optical fiber. There are many such distortions, including nonlinearities such as four wave mixing, but the principal ones are usually chromatic dispersion (CD) and polarization mode dispersion (PMD). PMD can vary over periods of minutes, sometimes much less and so needs adaptable control. Many complex solutions have been tried to compensate for PMD and CD with limited success. Solutions which correct the distortion in the optical domain involve expensive optical components.
Among the known PMD compensation techniques, electrical domain (post-detection) approaches are particularly attractive because of their potential for compact and cost-effective implementation in the chip sets at the receiver. Electronic equalizers using simple feedforward and decision feedback structures have been proposed for mitigating intersymbol interference (ISI) in optical communications by for example J. H. Winters and R. D. Gitlin, “Electrical signal processing techniques in long-haul fiber-optic systems,” IEEE Trans. Commun., vol. 38, pp.1439–1453, September 1990.
They have been recently implemented and tested at 10 Gb/s using integrated SiGe technology as analog equalizers for PMD mitigation as shown by H. Bülow, R. Ballentin, W. Baumert, G. Maisonneuve, G. Thielecke, and T. Wehren, “Adaptive PMD mitigation at 10 Gbit/s using an electronic SiGe equalizer IC,” in Proc. ECOC 1999, vol. II, pp. 138–139. However, it is noted that they do not deliver the performance gains typically expected and the optimization of filter coefficients adaptively, even with the simple and well known least mean squares (LMS) algorithm is still a challenging task at the high data rates at which optical systems operate. An example of an electronic compensator for a conventional 10 Gb/s optical transmission system has been announced by Santel Networks, of Newark, Calif. They claim that it provides a single solution for mitigating impairments from PMD and CD. It uses an equalizer for use on a directly detected electrical signal.
Since currently all installed high data rate systems use direct detection, the polarization and phase information is lost during detection. Diversity can provide advantages for PMD mitigation by making more efficient use of the available information. A known technique based on adaptive optics and diversity detection is described by B. W. Hakki, “Polarization mode dispersion compensation by phase diversity detection,” in IEEE Photon. Technol. Lett., vol. 9, Jan. 1997, pp.121–123,
where a polarization beam splitter (PBS) is used to split the signal into two orthogonal polarizations that are recombined in the electrical domain using an electrical delay line and a combiner. However this requires a polarization controller, which is likely to be expensive and bulky if they are to respond fast enough. Another diversity detection scheme that is based on fixed optics is presented in H. Bülow, “Equalization of bit distortion induced by polarization mode dispersion,” in Proc. NOC 1997, pp. 65–72 in which three polarizations are extracted from the optical signal to be recombined in the electrical domain. In this scheme the three receiver photodetector signals are adaptively weighted by different weighting factors and then superimposed. With only 3 detectors arranged for maximum polarization diversity, with polarization states uniformly distributed on an equatorial plane of the Poincaré sphere, there will be an orientation of the fibre's principal states for which there is no improvement in PMD impairment. If the detectors polarization states do not have this maximal separation, there will be input states with a substantial noise penalty, even in the absence of PMD.
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 14, NO. 4, APRIL 2002 465 “A Novel Polarization Diversity Receiver for PMD Mitigation” by A. O. Lima, I. T. Lima, Jr., Student Member, IEEE, T. Adali, Senior Member, IEEE, and C. R. Menyuk, Fellow, IEEE shows another polarization diversity receiver using simple fixed optics and electronics for incorporating equalization into the diversity receiver structure. In this case, six polarizations are used, which can be represented as three pairs, each diametrically opposed on the Poincaré sphere, and on three mutually perpendicular axes. Six detectors are used and the six electrical signals are fed to a transversal filter.